Musculoskeletal Disorders.
Musculoskeletal disorders are recognized as among the most significant human health problems that exist today, costing society an estimated $254 billion every year, and afflicting one out of seven Americans. It is expected that the number of individuals with musculoskeletal disorders will increase over the coming years, as our population ages. Yet, in spite of the enormous magnitude of this problem, there is still a lack of bone replacement material that is appropriate for restoring lost structure and function, particularly for load bearing applications. This problem has resulted in a need for improved and mature biomaterials for load-bearing implants.
Natural synovial joints (e.g., hip, knee or shoulder joints) are complex and delicate structures capable of functioning under critical conditions. Their performance is due to the optimized combination of articular cartilage, a load-bearing connective tissue covering the bones involved in the joint, and synovial fluid, a nutrient fluid secreted within the joint area (Pillar '75; Mow '91). Unfortunately, human joints are prone to degenerative and inflammatory diseases that result in pain and joint stiffness. Primary or secondary osteoarthritis (osteoarthrosis), and to a lesser extent rheumatoid arthritis (inflammation of the synovial membrane) and condromalacia (softening of cartilage), are, apart from normal aging of articular cartilage, the most common degenerative processes affecting synovial joints (Dowson '92; Ardill '95). In fact, 90% of the population over the age of 40 suffers from some degree of degenerative joint diseases (Schumacher '88). Premature joint degeneration may arise from deficiencies in joint biomaterial properties, from excessive loading conditions, or from failure of normal repair processes. The explicit degenerative processes are not yet completely understood. Though minor surgical treatments are performed to provide temporary relief to ailing patients, the ultimate need is to replace the dysfunctional natural joints by ceramic, metal or polymer-based artificial materials by means of what is known in the art as ‘total joint replacement’ (TJR) surgery.
Stress shielding. Bones in normal, healthy condition carry external joint and muscular loads by themselves. Following the insertion of orthopedic screws/implants, the treated bone will share its load-carrying capacity with the screws/implants. Thus the same load that had been originally born by the bone itself will now be carried by the ‘composite’ new structure. For load bearing screws and implants, the clinically available devices are metallic and therefore significantly stiffer (elastic modulus of around 110 GPa for titanium and over 200 GPa for steel) than the adjacent bone (modulus of 1-20 GPa), and internal loads will mainly be supported by the screws that are now ‘shielding’ the bone from carrying the normal mechanical stresses. This ‘stress shielding’ effect alters the normal stress stimuli for bone growth, and in accordance with Wolff's law, the reduction of bone stresses relative to the natural situation causes bone to adapt itself by reducing its mass in a process of resorption around the implant. The relationship between implant flexibility and the extent of bone loss has been established in clinical patient series and animal experiments and confirm that changes in bone morphology are an effect of stress shielding and a subsequent adaptive remodeling process. This resorption/bone loss effect will cause micromotion of the screws/implants in response to external loads and could further damage the interfacing bone layer and anchorage performances subsequent to possible loosening of the screw/implant (Gefen '02). Early loosening of the screws/implants can not only delay or damage the healing process, but can also endanger adjacent anatomical structures and can even require surgery for the immediate removal of the failed implants (Lowery '98). Such aspects inevitably impose a prolonged and painful rehabilitation process on patients, as well as substantial treatment costs. Animal (e.g., canine) experiments with screw and plate fixation systems have shown that cortical and trabecular bone losses are reduced if a reduced-stiffness implants with identical geometrical designs are used (Pillar '79; Tomita '87).
Choice of Materials for Orthopedic Implants.
The choice of material for each component of such an implant depends on the design, size and required strength of the system. For total hip (THR) and total knee (TKR) joint replacements surgeries, metals are considered as the best candidate due to their higher load bearing capabilities and higher fatigue resistance (Cohen '79). The requirements for modern day metallic implants, especially for total joint replacement can be broadly categorized as follows (Hoeppner '94): (1) superior biocompatibility between the material and surrounding environment with no adverse cytotoxicity and tissue reaction; and (2) the mechanical and physical properties necessary to achieve the desired function. Some desired properties are, for example, low modulus, high strength, good ductility, excellent corrosion resistance in the body fluid medium, high fatigue strength and good wear resistance.
The above criteria are met by only a handful of metals and alloys. In the past, only stainless steel (e.g., 316 and 316L) and cobalt based alloys (e.g., CoCrMo) were considered suitable for metallic implants. Wrought and lightly cold-worked 316 stainless steels are even now used for making the femoral component in the art-recognized ‘trapeziodal-28 total hip replacement’ surgery. Likewise, the femoral component of the art-recognized ‘total Condylar Prosthesis’ of the knee is made from the investment casting of the Cobalt Stellite 21 alloy. However, titanium and its alloys started gaining popularity as implant materials in the early 1970's because to their lower modulus, superior tissue compatibility and better corrosion resistance (Dowson '92).
Titanium and its alloys have been widely used for orthopedic and dental implant applications primarily due to their excellent combination of enhanced strength, lower modulus, good ductility, enhanced corrosion resistance, and good biocompatibility as compared with stainless steels and cobalt-based alloys. Commercially pure (cp) titanium was the first to be used. Though cp-Ti exhibited better corrosion resistance and tissue tolerance as compared to stainless steel, cp-Ti's rather limited strength confined its applicability to specific parts such as hip cup shells, dental crown and bridges, endosseous dental implants, pacemaker cases and heart valve cages (Wang '96; Lee '02). However, while the use of titanium-based alloys has been quite beneficial for such implants, high stiffness and high density of the alloy compared to natural bone is still a problem causing ‘stress-shielding.’ To improve the strength for load bearing applications such as total joint replacements, the alloy Ti-6Al-4V ELI (i.e., with extra low interstitial impurity content) was chosen as a candidate biomaterial for surgical implants in the late 1970's. Ti-6Al-4V is one of the most widely used Ti alloys and exhibits excellent corrosion resistance, low density, good biocompatibility, and excellent mechanical properties, including high strength and low modulus. Ti-6Al-4V has an elastic modulus of ˜110 GPa that is only about half that of 316L stainless steel (˜200 GPa) and CoCrMo alloys (˜210 GPa). The mechanical properties of Ti-6Al-4V are critically dependent on its microstructure and can consequently be tailored by thermo-mechanical processing.
Porous Metals.
Despite the great progress that has been achieved in orthopedic biomaterials, fixation of implants to the bone host remains a problem. Mismatch of Young's moduli of the biomaterials and the surrounding bone has been identified as a major reason for implant loosening following stress shielding of bone (Robertson '76). However, the implanted material must be strong enough and durable to withstand the physiological loads placed upon it over the years. A suitable balance between strength and stiffness has to be found to best match the behavior of bone. One consideration to achieve this has been the development of materials that exhibit substantial surface or total bulk porosity in medical applications. The fabrication of porous materials for biomedical applications has been actively researched since 1972 (Weber '72) in which osseointegration was shown in porous metals. Numerous investigations into porous materials where subsequently initiated involving porous ceramic, polymeric, and other metallic materials. Although ceramics portray excellent corrosion resistance, they cannot be employed as load bearing implants due to their inherent brittleness. Similarly, porous polymeric systems cannot sustain the mechanical forces present in joint replacement surgery. This led researchers to focus on porous metals, based on orthopedic metallic materials, as a consequence of their superior fracture and fatigue resistance characteristics, which are required for load-bearing applications. Ryan et al. recently published an excellent review on this subject (Ryan '06).
Boblyn et al. (Boblyn '90) performed an experiment on bilateral non-cemented total hip arthroplasties in canine models to determine the effect of stem stiffness on stress-related bone resorption. Two partly porous femoral implants of substantially different stiffness were designed for direct comparison. One was manufactured from Co—Cr alloy, the other from titanium alloy, but modified internally by drilling a hole that extended from the stem tip to within 5 mm of the shoulder, which transformed it into a hollow cylinder. Femora with the flexible stems consistently showed much less bone resorption than those with the stiff stems. Quantitative analysis of paired cross-sections indicated an average of 25-35% more cortical bone area in femora that received low stiffness hollow cylindrical stems.
Titanium and its alloy (Ti6Al4V) have elastic moduli less than 50% of that in Co—Cr implants so that their use would help reduce the extent of stress shielding. Although fabrication of implants from materials with lower elastic moduli can reduce stress shielding the stiffness mismatch to bone is still substantial (Otani '92). The clinical literature of the past 30 years records a variety of approaches to this end and several researchers have performed studies aimed at clarifying the fundamental aspects of interactions between porous metals and hard tissue.
Surface Modification of Load Bearing Metal Implants.
Surface modification is a common approach to increase bioactivity of load bearing metal implants. Either metal on metal or hydroxyapatite based ceramic coatings are most common. These coatings are commonly applied to metal surfaces using thermal spraying techniques such as plasma spraying, flame spraying, and high-velocity oxy-fuel (HVOF) combustion spraying (Berndt, '90). Reproducibility and economic efficiency of the thermal spraying techniques have an outstanding advantage. However, these methods present poor coating-substrate adherence and lack of uniformity of the coating in terms of morphology and crystallinity (Brossa, '93). In clinical applications, HA coatings prepared by plasma spraying techniques were found to ‘flake away’ from the surface of substrate surface after implantation in the body (Berndt, '90).